What are the key considerations for simulating turbomachinery with tip gaps?

Turbomachines often feature clearance gaps between rotating blade tips and the stationary casing. Tip leakage flow occurs through these gaps, producing substantial losses. The magnitude of these losses is approximately linearly proportional to the gap height, with the proportionality constant (leakage loss slope) depending on the blade design.

Turbine tip leakage flows account for approximately one-third of the total loss in a turbine stage. Tip leakage loss increases roughly linearly with the height of the tip gap and incurs a penalty of 1-3 percent or more in stage efficiency when the gap height equals 1 percent of the blade span. Additionally, the over-tip flow can enhance heat transfer in the tip region.

The flow through the gap can be modeled using a vena contracta with a coefficient of discharge. A separation bubble forms if the pressure side gap corner is sharp. However, the flow reattaches before exiting the gap if the blade tip thickness is greater than approximately four times the gap height.

Two primary blade designs exist: shrouded and unshrouded. Unshrouded blades have a gap between the tip and the casing, where pressure drives flow from the pressure side to the suction side. This leakage flow creates losses within the gap and also upon mixing with the mainstream flow. Shrouded blades, on the other hand, have a seal-forming endwall at the blade tips, minimizing leakage flow. The leakage flow in unshrouded blades can significantly impact the efficiency of turbomachines. Understanding the dynamics of tip leakage flow is crucial for optimizing the performance of turbomachines.

In the case of a thick blade, the flow mixes and reattaches in such a way that the separation bubble does not extend across the entire blade thickness, unlike in the case of a thin blade. For both thick and thin blades, the vortex and its direction of rotation are on the suction side of the blade passage.

When simulating tip-leakage flow, several important points should be taken into consideration.

 Key factors to keep in mind include:

1. Ensuring the tip height is accurate and matches the design specifications.
2. Verifying that the tip gap is fully meshed, free from holes, and negative volumes.
3. Maintaining a proper boundary layer thickness on the blade tip.
4. Achieving sufficient resolution to effectively capture the leakage vortex core.
5. Ensuring a smooth mesh transition between the main passage and the tip gap.

 Are there any additional important points that others can contribute based on their experience and research?